1. Field of the Invention
The present invention relates to an hydraulic apparatus for an industrial vehicle such as a fork lift truck or the like. More particularly, the present invention relates to a hydraulic apparatus which is adapted for controlling a capacity of a hydraulic pump for cargo handling and power steering operations.
2. Description of Related Art
An ordinary hydraulic apparatus installed on a fork lift truck is provided with a flow dividing valve which is disposed in an outlet pipe line of a constant capacity type hydraulic pump operated by an engine. A flow of a pressurized hydraulic oil is divided by the flow dividing valve into a flow required for a power steering hydraulic circuit and a balance flow for a cargo handling hydraulic circuit, and the pressurized hydraulic oil supplied to the cargo handling hydraulic circuit is then supplied to either a lifting cylinder or a tilting cylinder by operating a cargo handling control valve whenever it is needed.
In the above-described conventional hydraulic apparatus, part of the pressurized hydraulic oil flow discharged from the constant capacity type hydraulic pump, namely, all of the flow other than the flow required for the power steering hydraulic circuit is supplied to the cargo handling hydraulic circuit. Hence, when the cargo handling operation is not under way, the pressurized hydraulic oil is returned to an oil tank as an excessive hydraulic oil simply passing through the cargo handling control valve. In addition, the discharge flow increases proportionally as the speed of the engine increases, and accordingly the circulation of the useless and excessive hydraulic oil is further accelerated. Therefore, the repeated circulation of the useless pressurized hydraulic oil leads to the power loss as well as the oil temperature increment, and it consequently causes the earlier degradation of sealing members and the like. Thus, the circulation of the useless pressurized hydraulic oil is one of the serious causes which makes the defects happen in the hydraulic apparatus.
The inventors of the present invention aimed at solving the problem, and proposed a hydraulic apparatus in advance in Japanese Patent Application No. 222331/1989 which is not laid-open at the time of this application. The hydraulic apparatus detects the revolution fluctuations of a pump driven by an engine, and detects whether the cargo handling operation is under way or not, whereby enabling an appropriate pump capacity control.
As illustrated in FIG. 7, the proposed hydraulic apparatus comprises a variable capacity type hydraulic pump 62 driven by an engine 61, a flow dividing valve 64 disposed in an outlet pipe line 63 of the pump 62 and adapted for dividing a flow of a pressurized hydraulic oil into a flow required for a power steering hydraulic circuit 71 and a balance flow for a cargo handling hydraulic circuit 72, and a flow switching valve 65 with a throttle disposed in the outlet pipe line 63 approaching the flow dividing valve 64 and pilot-operated by a pressure "P3" of the cargo handling hydraulic circuit 72, and a capacity control valve 66 pilot-operated by a differential pressure "P1-P2" between the pressures before and after the flow switching valve 65 and adapted for controlling a capacity varying mechanism 67 of the pump 62. Further, in the proposed hydraulic apparatus, a flow after being throttled by the flow switching valve 65 is set so that it slightly exceeds the flow required for the power steering hydraulic circuit 71.
However, in the proposed hydraulic apparatus described above, the flow switching valve 65 with a throttle adapted for producing the differential pressure "P1-P2" is employed in order to pilot-operate the flow control valve 66 which is adapted for controlling the capacity varying mechanism 67 of the pump 62. Hence, the pressurized hydraulic oil might be subjected to an excessive resistance when it passes through the flow switching valve 65, there might be fear for the pressure loss in the pressurized hydraulic oil accordingly, and eventually there might arise the power loss of the engine 61.